Catalytic reduction of nitrous oxide content in gases

ABSTRACT

A process and catalyst for reducing the nitrous oxide content in gas, which operates at relatively low temperatures, the activity of which is relatively insensitive to the presence of water vapour and which is highly resistant to hydrothermal degradation, is prepared from ferrierite exchanged with iron. Application to the treatment of gases with a low N2O content, such as gases resulting from plants for the manufacture of nitric acid, and of gases with a high N2O content, which are emitted during oxidations of organic compounds by nitric acid.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The invention comes within the general scope of the reduction of thecontent of greenhouse gases in gaseous effluents of industrial origindischarged to the atmosphere. It is a question here of lowering nitrousoxide N₂O in gaseous discharges.

(ii) Description of Related Art

For a long time, concern was only felt about the discharge of nitricoxides (NOx), which easily combine with water to form nitrous or nitricacids, the most spectacular sign of which is without doubt acid rain,with subsequent destruction of forests and damage to exposed monuments,and the most insidious signs of which are contamination of breathableair and its effect on public health. Awareness has now arisen of thesignificant contribution of nitrous oxide to enhancing the greenhouseeffect, with the risk of leading to climatic changes with uncontrolledeffects, and perhaps also of its participation in the destruction of theozone layer. Its removal has thus become a preoccupation of theauthorities and of manufacturers.

While the most significant sources of N₂O are the oceans, uncultivatedsoils, agriculture, the combustion of organic matter and the use offossil fuels, the chemical industry contributes some 5 to 10% ofemissions of this gas. Nitric acid plants, as well as plants for organicsynthesis employing nitric oxidation processes (production of adipicacid, of glyoxal, and the like), are the source of most discharges ofN₂O by the chemical industry (see, in this respect, Freek Kapteijn etal., Heterogenous Catalytic Decomposition of Nitrous Oxide, in AppliedCatalysis B, Environmental 9, 1996, 25-64).

For some years already, most nitric acid plants have been equipped withso-called DeNOx reactors, which operate satisfactorily in removingnitric oxides from their effluents. However, N₂O, which is essentiallyproduced during the oxidation of ammonia over the platinum gauzes of theburners, remains substantially constant between the outlet of theburners and the inlet of the DeNO_(x) reactor and is not lowered bypassage of the gases through this reactor (sometimes, it is evenslightly increased).

Provision has been made to reduce the N₂O content of the gaseouseffluents resulting from nitric oxidation processes in organic chemistryby catalytically destroying the nitrous oxide contained in the latterover a mordenite/iron catalyst (EP 0,625,369). However, on account ofthe large fall in its activity in the presence of steam in thetemperature range 350-450° C., this catalyst is not well suited tofunctioning with respect to dilute gases and ages badly, due to amediocre hydrothermal resistance.

It also turns out to be economically unsuited to the treatment of thetail gases from nitric acid plants, which, upstream of the expansionturbine, generally correspond to the following characteristics,

temperature: <400° C., N₂O content: between 500 and 1500 ppmv, NOxcontent: between 50 and 2000 ppmv, H₂O content: between 0.5 and 5%.

The economic optimization of the lowering of N₂O both in the gasesemitted by organic plants and by nitric acid plants involves thedevelopment of a catalyst which retains a good activity for thedestruction of N₂O at a temperature below 400° C. in the presence of NOxand of steam, and which has a sufficient hydrothermal stability at 600°C to withstand the temperature peaks to which it may be subjected undercertain circumstances in its use.

SUMMARY OF THE INVENTION

A solution corresponding to such specifications has just been found witha catalyst composed of agglomerates formed of 80 to 90% of aferrierite/iron assaying from 1 to 6% of iron, and preferably from 2 to4%, and of 20 to 10% by weight of an agglomeration binder (percentagesby weight with respect to the weight of the granule).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The ferrierite/iron is the active component of the catalyst according tothe invention. The structure of its crystal lattice is that offerrierite [RN=12173-30-7], that is to say a zeolite traversed by twosystems of channels, one parallel to the c axis of the structure, formedof channels with an elliptical cross-section (0.43 nm×0.55 nm) ofapproximately 0.18 nm² (18 Å²), the other parallel to the b axis and thec axis of the structure, with channels formed of 8-membered rings, with0.34×0.48 nm axes. There is no channel parallel to the a axis.Approximately spherical cavities, with an approximate diameter of 0.7nm, lie on these channels and are accessible only through the 8-memberedrings, i.e. via 0.43 nm×0.55 nm or 0.34 nm×0.48 nm pores. Theferrieritic structure is completely characterized by its X-raydiffraction diagram (for the interlattice distances, consult Breck “TheSynthetic Zeolites”, 1974 Edition, Table 4.45, p. 358).

This ferrierite/iron is obtained by subjecting a commercial ferrierite,of sodium/potassium type, to exchange with an aqueous solution of aniron salt, so as to obtain the desired iron content. The operatingprocedures are well known to a person skilled in the art. It ispossible, in particular, to carry out one or more exchanges by immersionin an iron salt solution or by column percolation, either of theferrierite powder itself or with respect to granules.

This exchange can be carried out either using a ferric salt solution orusing a ferrous salt solution. Use is advantageously made of ferroussulphate, which is a very low cost product and which does not introducechlorides, which are sources of corrosion, into the preparation.

Preference is given to the form exchanged with iron starting from theammonium form of ferrierite, which is obtained by subjecting acommercial ferrierite, the electrical neutrality of the crystallographiclattice of which is essentially produced by sodium and potassium alkalimetal ions, to an exchange with a solution of an ammonium salt. Theferrierite/iron obtained from the ammonium form of ferrierite exhibits,as characteristic, that of having a very low content of alkali metalions in the exchange position. It is the low content of potassium ions(less than 0.5% by weight) which analytically indicates this preferredform of the catalyst of the invention. The ferrierites/iron according tothe invention contain only 0.5 to 0.1% of potassium.

The catalysts according to the invention are shaped as agglomerates, apresentation which is necessary for reasons of minimization of thepressure drop as they pass through the catalyst bed. The agglomerationof zeolites is well known to a person skilled in the art. It is carriedout by forming a paste of the zeolite powder with a binder, generallyfluidified with water, often composed of a clay which is simultaneouslysufficiently plastic to be able to form the agglomerate as balls, usinga dish granulator, as pellets by moulding or as extrudates, using anextruder, and hardenable by calcination to give sufficient cohesion andhardness to the agglomerate. The clays used are kaolinites,attapulgites, bentonites, halloysite or mixtures of these clays.

It is also possible to use siliceous or aluminous binders. Inparticular, agglomeration with peptized aluminas gives very stronggranules, this method of agglomeration being possible here becauseferrierite is not degraded by the acidity of the binder.

After agglomeration, the granules are thermally activated. This meansthat they are subjected to a calcination carried out under air at atemperature of approximately 400° C., the role of which is both toharden the binder, to dehydrate it without hydrothermally degrading itand, in the case of ferrierites exchanged starting from an ammoniumform, to remove a large part of the ammonium ions and to bring thezeolite to the H form.

It is also possible to start by agglomerating the sodium/potassiumferrierite, then to harden it by calcination and to carry out exchangeson the agglomerate. After drying, a second calcination makes it possibleto bring the ferrierite/iron to the H form, if the ferrierite employedwas taken in the ammonium form.

This catalyst is the improved catalytic means of a process fordestroying N₂O, contained in a gas mixture, according to the overallreaction:

2N₂O→2N₂+O₂

This process, which is also one of the subjects of the presentinvention, consists in passing the gases to be purified, in which therange of concentrations of N₂O extends from 500 ppm to 50%, of H₂O from0.5 to 5% and of NO from 50 to 2000 ppm, through a catalyst bed placedin an axial or radial flow reactor maintained at a temperature ofbetween 350 and 600° C. In the treatment of a gas with a high N₂Ocontent and with an initial temperature of less than 350° C., as isgenerally the case in processes for organic synthesis by nitricoxidation, the initiation of the reaction can be facilitated bypreheating, during the start-up phase, the gas flow or the catalyst byan external means, the temperature of the catalytic bed subsequentlybeing self-supporting because of the exothermicity of the reaction. Incertain situations, in particular in the case of the treatment of gaswith a high N₂O concentration, heat exchangers or devices of quench typecan advantageously be immersed in the catalytic bed in order to controlthe temperature of the latter, it optionally being possible to use partof the heat to preheat the gas to be treated.

Contrary to other zeolitic catalysts, the ferrierite/iron according tothe invention retains a manifest activity with respect to N₂O in thepresence of water. This activity is very much enhanced in the presenceof NO, which is a very favourable factor because this synergy becomesmore significant for very low levels of NO, of the order of 50 ppm, andbecause the gases capable of such a treatment almost always contain suchtraces of NO.

The process according to the invention finds its application inparticular in the treatment of tail gases from nitric acid plants, bothbefore and after DeNOx treatment, which gases can have compositionswithin the following limits,

N₂O content: between 500 and 1500 ppmv, NOx content: between 50 and 2000ppmv, H₂O content: between 0.5 and 3%, oxygen content: approximately 2%,

the remainder being essentially composed of nitrogen.

The process can also be applied to the treatment of gases resulting fromplants for organic oxidation using nitric acid in organic chemistry, inparticular in the manufacture of adipic acid, of glyoxal and ofglyoxylic acid. These are gases with the approximate composition, beforeoptional dilution with air, as follows:

N₂O content: between 20 and 50% NOx content: between 50 and 5000 ppmv,H₂O content: between 0.5 and 5%, oxygen content: between 1 and 4%, CO₂content: approximately 5%,

the remainder being essentially composed of nitrogen.

EXAMPLES

In the following examples, which are nonlimiting but intended to give abetter understanding of the invention, the same catalytic test procedurehas been followed, which procedure comprises the preparation of thesample and the catalytic test proper.

a) Preparation of the Catalyst

The exchanged zeolite powder is dried in an oven at 100° C. and thenmixed with a silica sol, containing 40% by weight of SiO₂, in an amountsuch that the silica SiO₂ content with respect to the SiO₂+zeolite drycombination is 10%. The paste obtained is dried at 100° C. for 6 hoursand then reduced to a powder in a mortar. The powder is pelletized topellets with a diameter of 5 mm which are activated in an oven at 400°C. under air for 2 hours. After cooling, the pellets are crushed andsieved at 0.5-1 mm, this fraction constituting the catalyst.

b) Catalytic Test

It is carried out in a traversable stationary bed test unit (catatest)surrounded by heating shells regulated by PID, which brings thecatalytic bed to a temperature approximately 25° C. below theirset-point temperature. The reactor has a diameter of 15 mm. The catalystvolume employed is 10 cm³, i.e. a bed with a height of 57 mm.

The reaction gas is prepared from compressed air, from nitrogen and fromstandard gas, 2% N₂O in N₂, 2% NO in N₂. The water vapour content isadjusted by an air humidifier, according to the laws of vapour pressure.

N₂O analyses are carried out by infrared and NOx analyses bychemiluminescence.

The results are expressed as degrees of conversion of N₂O to N₂.

Example 1 Preparation of Various Ferrierite/iron Compositions

The ferrierite is supplied by Tosoh. Its Si/Al ratio is 8.85 and its Naand K contents, on a dry basis, after calcination at 1000° C. are 0.92%and 4.7% respectively. Taking into account its loss on ignition of 25%at 1000° C., its formula is

0.75K.025Na.AlO₂.8.85SiO₂.116H₂O

The direct ferric exchange is carried out as follows. 100 g of zeolitepowder are suspended, in a 1 litre round-bottomed glass flask, with 0.5l of molar aqueous ferric chloride (FeCl₃) solution (i.e. 8.1 g of FeCl₃per litre), namely with a volume of liquid/weight of dry solid ratio of5. The system is kept stirred at 60° C. for 4 hours. The exchangedzeolite is recovered by filtration on a filter funnel, washed bypercolation with 2 litres of demineralized water at ambient temperatureand then dried on a tray in a ventilated oven overnight.

The iron, potassium and sodium contents with respect to the dry product(1000° C.) are 2.7%, 2.8% and 0.16% respectively. These quantities canbe varied by adjusting the temperature, the duration of the exchangesand their number.

Time Fe³⁺ Ref. T° (h) exchanges Fe % Na % K % 1.1 60 4 1 2.7 0.16 2.81.2 60 4 1 3.8 0.1 2.7 1.3 80 4 3 7.7 <0.05 0.16

These products are subsequently named FERFe³⁺, Na, K form.

The ferric exchange on ferrierite exchanged beforehand with ammoniumions is carried out as follows.

A first exchange is carried out, on 100 g of the same zeolite as above,with 0.5 litre of an 800 g/l ammonium nitrate solution at a temperatureof 80° C. for 4 hours. The exchanged product is recovered, washed anddried as above. Its sodium content is less than 0.1% and its potassiumcontent less than 0.15%.

The ferric exchange is subsequently carried out as above but with twosuccessive exchanges. The continuation of the operation is the same asin Example 1. A ferrierite/iron is obtained for which the iron,potassium and sodium contents are 2.2%, 0.15% and less than 0.1%respectively. These quantities can be varied by adjusting thetemperature, the duration of the exchanges and their number. Thefollowing were thus obtained

Time Number of Ref. T° (h) Fe³⁺ exchanges Fe % Na % K % 2.1 60 5 1 1.262.2 60 4 2 2.2 <0.05 0.15 2.3 80 4 1 3.2 <0.05 0.12 2.4 80 4 2 7 <0.05<0.05

These products are subsequently named FERFe³⁺, NH₄ form.

Example 2 Power of Conversion of N₂O of Ferrierites/iron³⁺ in gases witha low N₂O content

The test is carried out, according to the experimental procedureexplained above, on nitrogen enriched with

N₂O 1000 ppm O₂ 2%

at an hourly volumetric rate or HVR of 10,000 h³¹ ¹.

In addition, the gas may or may not contain nitrogen oxide NO or water.The specific conditions of the test are as follows

1: 375° C., NO = O, H₂O = 0 2: 375° C., NO = 1000 ppm, H₂O = 0 3: 375°C., NO = 1000 ppm, H₂O = 3% 4: 400° C., NO = 1000 ppm, H₂O = 3%

The following % conversion results are obtained

Conversion of N₂O to N₂, different conditions Test conditions Ref. Fe %1 2 3 4 Na, K 1.1 2.7  10%  50%  30%  42% form 1.2 3.8 14 50 20 45 1.37.7 35 75 34 72 H form 2.1 1.26 49 88 44 72 2.2 2.2 46 97 48 77 2.3 3.224 79 35 66 2.4 7 33 84 52 85

An excellent activity of the ferrierite/iron, H form, is observed.

Example 3 Power of Conversion of N₂O of Ferrierites/iron in Gases with aLow N₂O Content

The preceding operations are repeated but, instead of ferric chloride,the exchange is carried out with a ferrous salt, ferrous sulphateFeSO₄.7H₂O. The procedures are carried out equally in Na, K form and inNH4 form. The products of the FERFe²⁺, Na, K form, series:

Time Number of Ref. T° (h) Fe²⁺ exchanges Fe % Na % K % 3.1.1 80 4 1 1.80.25 3.2 3.1.2 80 4 3 4.1 0.2 1.8

and the products of the FERFe²⁺, NH₄ form, series:

Time Number of Ref. T° (h) Fe²⁺ exchanges Fe % Na % K % 3.2.1 80 4 1 1.7<0.05 0.15 3.2.2 80 4 3 5.46 <0.05 0.15

are thus obtained.

The results of the catalytic test are as follows, the conditions beingthose in the preceding example:

Conversion of N₂O to N₂, different conditions Test conditions Ref. Fe %1 2 3 4 Na, K 3.1.1 1.8  12%  90%  22%  43% form H form 3.2.1 1.7 31 9348 78 3.2.2 5.46 29 98 50 78

An excellent activity of the ferrierite/iron, form H, is observed. Thereis no substantial difference between the ferric and ferrous series.

Example III Conversion of N₂O—Comparison of Various Zeolites/iron

Various zeolites/iron, all exchanged in their NH₄ form starting fromferrous sulphate, are now compared with a ferrierite/iron²⁺, at ironassays in the region of 2%. The zeolite Y is a Y with an Si/Al ratio of20 and assays, after exchange, 1.8% of iron and <0.1% of sodium; thepentasil has an SI/Al of 13.5 and assays, after exchange, 1.6% of ironand <0.05% of sodium; the beta has an Si/Al of 12.5 and assays, afterexchange, 1.9% of iron and <0.05% of sodium; the mordenite has an Si/Alof 5.5 and assays, after exchange, 1.9% of iron and <0.05% of sodium.The ferrierite is the ferrierite with the reference 2.2 in Example 2.

Conversion of N₂O Test conditions Zeolite Iron % 1 2 3 4 Y 1.8 28 45 2238 Pentasil 1.6 7 62 14 30 Beta 1.9 47 98 21 44 Mordenite 2.4 8 91 22 42Ferrierite 2.2 46 97 48 77

It is found that only the ferrierite retains a significant activity inconversion of N₂O in the presence of water vapour.

Example IV Comparative Activities of a Mordenite/iron and of aFerrierite/iron in Gases with a High N₂O Content

The reduction in the N₂O content obtained with the precedingmordenite/iron containing 2.4% of iron is compared with that of twoferrierites, one containing 1.46% of iron and the other containing 3.37%of iron.

The conditions of the test are

N₂O 5% O₂ 5% HVR 10,000 h⁻¹ 5: 325° C., NO = 0 6: 325° C., NO = 1000 ppm7: 375° C., NO = 0 8: 375° C., NO = 1000 ppm 9: 425° C., NO = 0 10: 425°C., NO = 1000 ppm 11: 475° C., NO = 0 12: 475° C., NO = 1000 ppm

The degrees of decomposition below are recorded.

Conversion of N₂O to N₂ Test conditions Fe % 5 6 7 8 9 10 11 12Mordenite/ 2.4 0.1 0 0.8 14.3 6.8 21.6 35.2 65.8 iron Ferrierite/ 1.461.6 1.2 5.3 11.6 12.3 36.7 42.1  8.7 iron Ferrierite/ 3.37 0.8 3.2 1.113.2 5 93.4 49.3 99.9 iron

These results exhibit a higher level of conversion of N₂O with theferrierite.

Example V Ageing

The result of a comparative hydrothermal stability test between amordenite/iron with an Si/Al ratio of 5.5, H form, exchanged with ironto the level of 2.4% by weight, and a ferrierite/iron according to theinvention, an H form, exchanged with iron to the level of 2.2%(reference 2.2 in Example 1), is reported here.

The ageing was carried out by exposure of the catalysts to an air/watervapour mixture in a dried bed at 650° C. for 3 hours. The air issaturated with water vapour at 90° C.

The two catalysts are tested as above with respect to conversion of N₂O,the operating conditions being

N₂O 1000 ppm NO 1000 ppm O₂ 10% Temperature: 375° C. HVR 10,000 h⁻¹ 13:H₂O = 0 14: H₂O = 3%

The following results are obtained:

Conversion of N₂O to N₂ Test conditions 13 14 Mordenite Before ageing 9122 Mordenite After ageing 32 10 Ferrierite Before ageing 88 40Ferrierite After ageing 83 39

which results confirm the remarkable stability of the ferrierite/iron towater vapour.

Example VI Granules with an Aluminous Binder

In a first step, extrudates containing 20% of aluminous binder areformed as follows. An alumina of NG type, supplied by the companyCondea, is used for the manufacture of the agglomerated catalyst. In afirst step, it is peptized by continuously introducing, into a mixer,alumina at the rate of 15 kg/h and 5% by weight nitric acid with a flowrate of 0.16 l/min. 5 kg of the peptized alumina gel thus obtained aremixed with 10 kg of ferrierite powder, in the Na,K form, as supplied byTosoh (see Example 1), in a conventional powder mixer. The resultingmixture is fed to a mixer/extruder at the same time as 3 litres ofwater. The extruder is a device of Redco type from the company Aoustin,with a diameter of 5 cm, equipped at the outlet with a die formingextrudates with a diameter of 3.8 mm which are cut into elements with alength of 5 to 10 mm. The extrudates are subsequently transferred, witha thickness of approximately 15 mm, to a muffle furnace, through whichair passes, at 100° C. for 4 hours and then at 450° C. for 3 hours, inorder to confer a satisfactory mechanical strength on them.

200 g of these ferrierite extrudates are now introduced into a stainlesssteel basket in order to steep them in 1 litre of an 800 g/l ammoniumnitrate solution at a temperature of 80° C. for 3 hours, then to washthem by successive steepings (3) in 1 litre of demineralized water, andthen to dry them at 100° C.

Their sodium and potassium content on a dry basis (1000° C.) is 0.1%(Na) and 0.15% (K).

Exchange with iron is then carried out according to the same principlewith 1 litre of iron (Fe²⁺) sulphate solution containing 280 g/l ofFeSO₄.7H₂O at 80° C. for 3 hours, followed by washing by successivesteepings in 1 litre of demineralized water and by drying. The ironcontent on a dry basis (1000° C.) is 1.6%.

The catalyst thus prepared is subjected to the catalytic test describedabove in a reactor with a diameter of 25 mm. The catalyst volume is 25cm³, i.e. a height of approximately 5 cm. The catalytic test is appliedunder the conditions 1 to 4 of Example 2.

Conversion of N₂O to N₂ Test conditions Catalyst 1 2 3 4 Aluminousgranules 30% 89% 43% 72%

which are results highly comparable with those of Example 2.1.

What is claimed is:
 1. A process for reducing nitrous oxide content ingases comprising from 500 ppm to 50% of N₂O, from 0.5 to 5% of H₂O andfrom 50 to 2000 ppm of NO, comprising passing said gases through acatalyst bed, brought to 350/600° C., wherein the catalyst bed comprisesan agglomerate of: (i) 80 to 90% of ferrierite; and (ii) 20 to 10% of anagglomeration binder; wherein the ferrierite assays from 1 to 6% of ironby weight.
 2. The process according to claim 1 wherein said gases aregenerated by plants for production of nitric acid, which gases compriseN₂O: between 500 and 1500 ppmv, NOx: between 50 and 2000 ppmv, H₂O:between 0.5 and 5%, Oxygen: approximately 2%, and a remainderessentially comprised of nitrogen.
 3. The process according to claim 1wherein said gases are generated by plants for production of organicsubstances by nitric oxidation, which gases, before optional dilutionwith air, comprise N₂O: between 20 and 50%, NOx: between 50 and 5000ppmv, H₂O: between 0.5 and 5%, Oxygen: between 1 and 4%, CO₂:approximately 5%, and a remainder comprised essentially of nitrogen.